Sheet material, such as fabric, is commonly cut into patterns on electronically guided machines comprising an elongated table over which a cutting tool is moved in a desired pattern by means of an precision positional control mechanism. Such tables are typically provided with a perforated top, below which a vacuum is applied for the purpose of drawing a multiple ply stack of the sheet material against the tabletop, thereby retaining it in position while it is being cut. Should the multiple layers of sheet material be retained effectively, a consistent relationship can be maintained between the cutting tool and the stack, enabling sheets with accurately cut patterns to be obtained reliably. On the other hand, should the sheets within the stack move from their intended position, a flawed pattern is cut into the sheets, resulting in excessive waste of material. The efficacy of the vacuum-operated sheet retention system therefore has a direct bearing on the economics of the entire cutting process.
To assure that the lower layers of the stack are cut properly, the cutting blade must be permitted to pass below the lowest layer. In order to avoid damage to the surface of the table, it is the common practice to provide a supporting bed between the tabletop and stack of material being cut. This supporting bed must have certain physical properties, in order to serve its purpose effectively. First of all, it must provide a firm, relatively unyielding support beneath the stack of material being cut, to avoid undesirable stack movement beneath the blade and resultant pattern errors in or damage to the cut sheet material. Secondly, the supporting bed must not impede the vacuum which is applied beneath the tabletop. It must therefore be capable of having a substantial volume of airflow through it. Third, it should have a relatively high coefficient of friction and should present the largest possible surface area to the bottom sheet of the stack, in order to avoid slipping of the stack relative to the tabletop. Finally, the supporting bed must have an upper surface which resists the gouging action of the cutting blade, in order to maintain the uniformity of its surface and to minimize the frequency of replacement of the supporting bed.
Various materials have been utilized for the supporting bed. Most commonly, it is made of a sheet of polyethylene foam which is approximately one inch thick. Polyethylene foam provides a rather firm support for the stack of sheet material. However, being a closed cell foam it is impervious to air. Accordingly, it is the common practice to punch or drill interspersed vertical holes through the polyethylene foam sheet, and a substantial number of such holes is required (per unit of sheet surface area), in order to provide the vacuum at the surface of the polyethylene sheet. Typically, for a one inch thick sheet, the holes would be about 5/16 of an inch in diameter and would be at a center-to-center spacing of about 1.5 inches. However, such a density of holes substantially reduces the firmness and surface area of the supporting bed, and the expense involved in forming the holes substantially increases the cost of the supporting bed.
In addition, such a perforated supporting bed holds the fabric effectively only at the holes. Between the holes, there may be wrinkling or bunching of the fabric, and the fabric above the holes may be stretched or frayed when the blade passes into the hole. Both of these effects result in cutting errors or damage to the fabric. The use of a perforated polyethylene foam supporting bed therefore represents, at best, a compromise, which results in a serious limitation upon the height to which the sheet material may be stacked and, even then, a certain amount of undesirable movement of the stack and damage to the sheet material will occur during cutting. As a result, some portion of the sheets cut by the machine will be unacceptable and must be discarded.
It has also been suggested that the supporting bed be made of upright bristles. Although such a construction provides a substantial airflow, it hardly provides an adequately firm supporting surface, particularly when a relatively heavy sheet material is being cut. Furthermore, this relatively weak support deteriorates rapidly, as the bristles are damaged by the cutting blade, after repeated use, and the supporting surface they provide becomes uneven.
Polyurethane foam has been suggested as a covering material for the surface of a supporting bed, because it exhibits the property of "healing" or recovering instantaneously from surface nicks inflicted by a sharp implement. Polyurethane foams may be either of the open or "tight" cell variety. In polyurethane foams, the individual cells are formed from a 3-dimensional skeletal structure comprising interconnected strands. Membranes or windows are attached to the strands and serve to divide or partition individual cells. In general the skeletal structure is substantially thicker than the windows or membranes. In so called "open cell" foams, a substantial number of the windows or membranes are broken or ruptured (even though they are still attached at their peripheral edges to the skeletal strands). Some small percentage of the windows may not be attached to the strands at the edges, or may be missing altogether, and this permits a limited air flow through the foam mass. Tight cell urethane foams have essentially all of the cellular windows or membranes intact (unbroken) and attached to skeletal structure of the foam. The use of polyurethane has been substantially limited, however, for essentially the same reasons as polyethylene.
"Reticulated" materials are also known to the art. Such materials have the cell membranes or windows partially or totally destroyed. These reticulated materials are prepared from the cellular materials of the prior art. Reticulated foam materials generally permit the passage of substantially greater volumes of air, in comparison to open or tight foam materials. Such reticulated foams generally have higher porosity than comparable "open" or "tight" cell foams Thus, in these reticulated materials, the primary support is supplied by the skeletal structure, since the cell membranes have been partially or totally eliminated. Examples of such reticulated materials extensively used by the prior art are the membrane destroyed or reticulated polyurethane foams which are employed in various filtering and detraining applications and as garment liners. Such reticulated foam materials and their process of manufacture are disclosed, for example, in U.S. Pat. Nos. 3,175,025 and 3,175,030 granted to Henry C. Geen on Mar. 23, 1965.
Reticulated materials of the flexible polyurethane type, have been in use for some time, owing to their porosity and softness as compared to non-reticulated flexible polyurethane cellular materials. However, attempts to use such materials in the supporting bed of a cutting machine have proven unsuccessful, because such materials offer virtually no support to the stack of sheet material while it is being cut and because the reticulated foam tends to collapse when the vacuum is applied.
Broadly, it is an object of the present invention to provide a supporting bed for supporting a stack of sheet material in a cutting machine of the type retaining the material by means of a vacuum applied from below the supporting bed, which supporting bed overcomes the disadvantageous and shortcomings of prior devices of this type.
It is specifically an object of the present invention to provide a supporting bed of the type described which is constructed so as to permit relatively free airflow therethrough, so as not to impede the holding action of the applied vacuum.
It is a further object of the present invention to provide a supporting bed of the type described which is constructed so as to provide a relatively firm, unyielding slip-free and continuous support for a stack of sheet material being cut on a pattern cutting machine.
It is yet another object of the present invention to provide a supporting bed of the type described which is substantially resistant to surface gouging inflicted by a sharp instrument.
It is yet another object of the present invention to provide a supporting bed of the type described which is reliable and convenient in use, yet relatively inexpensive and simple in construction, and requires a minimum of preparation and maintenance.
It is also an object of the present invention to provide a process for manufacturing a supporting bed of the type described.
In accordance with the present invention, a supporting bed is manufactured from a sheet of reticulated foam material which has been compressed under heat and pressure so as to be permanently reduced to approximately 10-35% of its initial thickness. The degree of compression, the temperature and compression time, and the porosity of the reticulated foam starting material are selected to provide particular airflow and firmness characteristics for the finished supporting bed. Preferably, a 1-inch thick sheet of the material should permit an airflow of at least 1.5 cfm through an area four inches square, with a pressure drop between the surfaces of the sheet material corresponding to 1/2 inch of water, and it should be sufficiently firm so that compressing a 1-inch thick sheet by one quarter of its thickness requires a pressure in excess of 1.5 psi. In accordance with a preferred embodiment of the invention, the starting material is a reticulated polyurethane foam of the- graft polyether type. The presently most preferred foam has a porosity of 30 pores per inch and a sheet thickness of 5 inches. The sheet of foam material is compressed to a thickness of one inch and retained under pressure for 10 minutes at a temperature of about 400.degree. F.